1. Field of the Invention
This invention relates to an automatic brake booster as an automobile brake and the like and more particularly to improvement in its valve mechanism.
2. Description of the Related Art
A heretofore known automatic brake booster typically comprises a valve body slidably provided within a shell, power pistons provided in the valve body, a constant pressure chamber and a variable pressure chamber formed in front of and behind each power piston, a valve mechanism provided for the valve body, an input shaft for switching the passages of the valve mechanism by reciprocally moving a valve plunger constituting the valve mechanism, an output shaft slidably provided in the valve body, and a reaction disc and a plate plunger installed between the base of the output shaft and the valve plunger, the valve mechanism including a first valve seat formed in the valve body, a sleeve slidably fitted into the valve body, a second valve seat formed on the sleeve, valve discs each seated on the first and second valve seats, drive means for switching the passages by displacing the sleeve frontward, and a valve plunger which is relatively displaceably provided for the valve body and the sleeve, and is interlocked with an input shaft and used for switching the passages by displacing the sleeve frontward when the input shaft is moved forward.
In such conventional automatic brake boosters, solenoids are generally employed as drive means.
However, the size of such a solenoid will have to be increased in order to secure greater magnetic force. In conventional valve mechanisms, it has been attempted to reduce the size and weight of the solenoid by decreasing the difference between the magnetic force of the solenoid and the urging force of a urging spring and also decreasing the difference between the urging force of the urging spring and force for urging a sleeve frontward.
For example, the force for urging the sleeve frontward has been relied upon pressure difference acting on a sleeve and a valve disc to be seated on a second valve seat that is formed on the sleeve.
As the smaller the solenoid gap is set, the greater the thrust acting on the sleeve becomes, the solenoid gap has heretofore been set smaller so that sufficient operating response performance may be acquired during the automatic braking operation in consideration of the magnetic force of a small-sized solenoid.
However, the opening amount of the second valve seat will become smaller if the solenoid gap is set smaller and consequently it has been pointed out that the operating response performance of the conventional automatic brake booster is bad at the abrupt braking time during the normal braking operation because the opening amount of the second valve seat has been set equal during the normal braking operation and the automatic braking operation both.
As braking counterforce has been arranged so that it is not transmitted to the sleeve in the conventional automatic brake booster, an attempted has been made to regulate the opening amount of the second valve seat by varying the magnetic force of the solenoid. However, the second valve seat will become fully opened even when the magnetic force of the solenoid is reduced and this has made it difficult to control the braking output. Consequently, the disadvantage is that the braking output becomes excessive in a relatively low speed zone.
Although it is reasoned that the braking counterforce is transmitted to the sleeve by bringing the sleeve into contact with the plate plunger in order to solve the foregoing problem, the opening amount of the second valve seat decreases because the braking counterforce transmitted from the plate plunger becomes too great relative to the magnetic force of the solenoid (thrust of the sleeve) with such an arrangement as mentioned above. Therefore, there still exists a drawback in that an insufficient braking output results in bad operating response performance.